Set sharding key for the fork_networks table to organization_id

Sharding keys need to be set for the tables: fork_networks to organization_id.

Deliverables

• Add organization_id to fork_networks, NULL, has no default, has a foreign key to organizations: !182898 (merged)
• Add support in code for new column organization_id on fork_networks: !184033 (merged)
• Backfill fork_networks records with organization_id: !184051 (merged)
• moved to #535998 (closed)
  • Finalize BBM above
  • Add validations in code for new column organization_id on fork_networks after backfill: !184826 (merged)
  • Add the NOT NULL constraint (after backfill migration is done) to move closer to sharding goal
• Add organization_id sharding key (this issue)